Rhodium catalyst regeneration by thermal treatment

ABSTRACT

This invention relates to the continuous production of alkane polyols by the reaction of hydrogen and oxides of carbon in a homogeneous liquid phase mixture containing a rhodium carbonyl complex catalyst wherein the improvement comprises periodically lowering the temperature of the liquid phase mixture and thereafter raising it to about its former value.

This invention is concerned with a process for improving the productionof alkane polyols, as well as a variety of other chemicals, inparticular, methanol, from the reaction of synthesis gas in ahomogeneous liquid phase mixture containing a rhodium carbonyl complexcatalyst.

There are described in U.S. Pat. No. 3,833,634 issued Sept. 3, 1974, andU.S. Pat. No. 3,957,857 issued May 18, 1976, processes for reactinghydrogen and oxides of carbon in the presence of rhodium carbonylcomplex catalysts. The conditions employed in those processes involvereacting a mixture of an oxide of carbon and hydrogen with a catalyticamount of rhodium in complex combination with carbon monoxide, at atemperature of between about 100° C. to about 375° C. and a pressure ofbetween about 500 psia to about 50,000 psia.

In addition to the aforementioned U.S. Patents, the following U.S.Patents amplify the development of the processes for making alkanepolyols from gaseous mixtures containing hydrogen and carbon monoxide inthe presence of a rhodium carbonyl complex catalyst.

    ______________________________________                                        U.S. Pat. No. 3,878,292                                                                         Patented April 15, 1975                                     U.S. Pat. No. 3,878,290                                                                         Patented April 15, 1975                                     U.S. Pat. No. 3,878,214                                                                         Patented April 15, 1975                                     U.S. Pat. No. 3,886,364                                                                         Patented May 27, 1975                                       U.S. Pat. No. 3,940,432                                                                         Patented February 24, 1976                                  U.S. Pat. No. 3,929,969                                                                         Patented December 30, 1975                                  U.S. Pat. No. 3,952,039                                                                         Patented April 20, 1976                                     U.S. Pat. No. 3,948,965                                                                         Patented April 6, 1976                                      U.S. Pat. No. 3,944,588                                                                         Patented March 16, 1976                                     U.S. Pat. No. 3,957,857                                                                         Patented May 18, 1976                                       U.S. Pat. No. 3,968,136                                                                         Patented July 6, 1976                                       U.S. Pat. No. 3,974,259                                                                         Patented August 10, 1976                                    (formerly U.S. Ser. No. 455,380,                                              filed March 27, 1974)                                                         U.S. Pat. No. 3,989,799                                                                         Patented November 2, 1976                                   (formerly U.S. Ser. No. 455,379,                                              filed March 27, 1974)                                                         U.S. Pat. No. 4,013,700                                                                         Patented March 22, 1977                                     (formerly U.S. Ser. No. 526,942,                                              filed November 25, 1974)                                                      U.S. Ser. No. 488,139                                                                           Filed July 12, 1974                                         (now abandoned)                                                               U.S. Ser. No. 506,862                                                                           Filed September 17, 1974                                    (now abandoned)                                                               U.S. Pat. No. 4,001,289                                                                         Patented January 4, 1977                                    (formerly U.S. Ser. No. 506,864,                                              filed September 17, 1974)                                                     U.S. Ser. No. 506,865                                                                           Filed September 17, 1974                                    (now abandoned)                                                               U.S. Ser. No. 615,093                                                                           Filed September 19, 1975                                    (now abandoned)                                                               U.S. Ser. No. 537,885                                                                           Filed January 2, 1975                                       (now abandoned)                                                               U.S. Ser. No. 618,023                                                                           Filed September 30, 1975                                    (now abandoned)                                                               U.S. Ser. No. 618,061                                                                           Filed September 30, 1975                                    (now abandoned)                                                               U.S. Ser. No. 618,021                                                                           Filed September 30, 1975                                    U.S. Ser. No. 727,646                                                                           Filed September 29, 1976                                    (now abandoned)                                                               U.S. Ser. No. 782,986                                                                           Filed March 30, 1977                                        (now U.S. Pat. No. 4,111,975,                                                 patented September 5, 1978).                                                  ______________________________________                                    

The processes described in the foregoing patents are conducted in ahomogeneous liquid phase, which means that the catalyst, the reactionproducts and the promoter, if present, are in a solution. The solutiontypically requires the presence of a solvent, primarily to keep therhodium catalyst in solution. However, rhodium carbonyl complexes varyin structure depending upon many variables, such as, temperature,solvent, promoter, and the carbon monoxide and hydrogen pressureemployed, such that a complex which may be extremely stable in solutionat one set of operating conditions could precipitate out of solution,for example, at a different pressure, or temperature, or in the presenceof a different promoter or solvent. In addition, it has been observedthat even at seemingly constant operating conditions, rhodium catalystis continuously lost from solution during periods of extended operationresulting in reduced productivity of reaction product. Consequently, itwas heretofore necessary to periodically add soluble rhodium compoundsto form new catalyst in order to maintain the desired level of reactionproductivity.

In large-scale industrial processes, catalyst losses are generallyunacceptable, particularly when the catalyst contains rhodium metalwhich is currently priced at about $500 per troy ounce. Thus, forexample, in the processes described in the aforementioned patents, it isestimated that rhodium losses on the order of about 0.1% by weight on aper pass basis would be sufficient to make the process uneconomical.Accordingly, the commercialization of these processes necessitates thatthe loss of rhodium metal be kept well below 0.1%, by weight, if thecost of the reaction product is not to exceed the cost of such productswhen manufactured by other competitive processes.

There is described herein an improved process for continuously formingalkane polyols at high levels of productivity from the reaction ofhydrogen and oxides of carbon in a homogeneous liquid mixture containinga rhodium carbonyl complex catalyst wherein the temperature of saidliquid phase mixture is periodically lowered and thereafter raised toabout its former value.

It has been found that the reaction productivity which generallydeclines over extended periods of time during normal operation can berestored to about its original level by lowering the normal reactiontemperature, generally in the range of about 210° C. to about 300° C.,for a period of time sufficient to at least partially convert therhodium to a more active catalytic state and thereafter raising thetemperature to about its former value. The loss of reaction productivitywhich occurs during extended periods of operation is believed to be dueto conversion of rhodium to an inactive form, for example, precipitationof rhodium from the catalyst solution. Experimental evidence indicatesthat rhodium which is lost or precipitated from solution isresolubilized during such period of time that the reaction temperatureis temporarily lowered thereby resulting in an increased level ofproductivity when the reaction is conducted, once again, at the normalreaction temperature.

To effect resolubilization of rhodium, the reaction temperature islowered in accordance with the invention for a period of time sufficientto at least partially resolubilize rhodium precipitated from solution.The reaction temperature may be lowered by about 10° C. to 150° C.,preferably about 10° C. to 30° C., relative to its normal operatingtemperature, although at a minimum, lowering the temperature even lessthan 10° C. will also effect a partial recovery of rhodium precipitatedfrom solution. As a general rule, it is preferred to lower the reactiontemperature as little as possible in order to minimize the energyrequirement associated with cooling and reheating the reactor inaccordance with the practice of the invention.

The amount of time required to convert the rhodium to a more activecatalytic form at the lower reaction temperature will generally varyfrom the order of about several minutes to about 40 hours depending uponthe extent to which the reactor is cooled and other reaction variables,such as, pressure and the H₂ /CO mole ratio. As a general rule, the morethe reactor is cooled relative to its normal operating temperature, theshorter the time period necessary to effect recovery of rhodiumprecipitated from the reaction solution. Likewise, the higher thereaction pressure, the shorter the required time period to effect suchresolubilization of rhodium.

Economic consideration will generally dictate the most convenient modeof operating the process of the invention. As described above, oneembodiment of the process is characterized by a periodic interruption ofthe reaction whereby the reaction temperature is periodically lowered inorder to effect at least a partial resolubilization of rhodiumprecipitated from solution and thereafter raised to about its formervalue, the reaction productivity being thus increased from its reducedlevel. The resolubilization of rhodium and/or its conversion to a moreactive catalytic form in such periodic manner may be effected wheneverthe reaction productivity declines to below a predetermined value, or atfixed intervals of time, whichever is more practical from an operationalstandpoint. Alternatively, the process may be operated continuously,i.e., without interruption of the reaction, at near constantproductivity. In accordance with this embodiment of the invention, aportion of the liquid phase mixture has its temperature lowered outsideof the reactor (or reaction zone) on a continuous or intermittent basis,depending upon the rate at which the rhodium in solution would otherwisetend to precipitate or become unstable, and then reintroduced into thereactor which is maintained at constant temperature. A portion of thecatalyst solution is thus withdrawn from the reactor, preferablycontinuously, and cooled to the desired temperature, and thenreintroduced into the reactor (or reaction zone). Such thermal treatmentof the liquid phase mixture external to the reactor is intended toprevent rhodium in solution from reaching a condition where it will tendto precipitate from the catalyst solution or otherwise become convertedto a less active form thereby enabling long-term uninterrupted operationof the process at near constant reaction productivity.

The rhodium carbonyl complex catalysts suitable for use herein may be inthe form of rhodium carbonyl clusters. P. Chini, in a review articleentitled "The Closed Metal Carbonyl Clusters" published in Review(1968), Inorganica Chimica Acta, pages 30-50, states that a metalcluster compound is "a finite group of metal atoms which are heldtogether entirely, mainly, or at least to a significant extent, by bondsdirectly between the metal atoms even though some non-metal atoms may beassociated intimately with the cluster". The rhodium carbonyl clustercompounds contain rhodium bonded to rhodium or rhodium bonded to anothermetal, such as cobalt, and/or iridium.

The preferred rhodium carbonyl cluster compounds are those which containrhodium-rhodium bonds. These compounds desirably contain carbon andoxygen in the form of carbonyl (--CO), in which the carbonyl may be"terminal", "edge-bridging", and/or "face-bridging". They may alsocontain hydrogen and carbon in forms other than carbonyl.

The structures of the rhodium carbonyl clusters may be ascertained byX-ray crystal diffraction, nuclear magnetic resonance (NMR) spectra, orinfrared spectra as disclosed in the article entitled "Synthesis andProperties of the Derivatives of the [Rh₁₂ (CO)₃₀ ]²⁻ Anion" by P. Chiniand S. Martinengo; appearing in Inorganica Chimica Acta, 3:2 pp 299-302,June (1969). Of particular analytical utility in the present inventionis the use of infrared spectroscopy which allows for characterization ofthe particular rhodium carbonyl complex present during the operation ofthe process of the present invention.

The precise role of the rhodium carbonyl complexes, such as the rhodiumcarbonyl clusters, in the reaction of hydrogen with oxides of carbon toproduce polyhydric alcohols is not fully appreciated at present. Underthe reaction conditions of the present process the carbonyl complexesare believed to be anionic in their active forms.

Infrared spectra under reaction conditions of the present process haveshown Rh(CO)₄ ⁻, Rh₁₃ (CO)₂₄ H₃ ⁻², Rh₆ (CO)₁₅ H⁻, Rh₁₃ (CO)₂₄ H₂ ⁻³,and [Rh₁₂ (CO)₃₄₋₃₆ ]²⁻ anions, and other rhodium clusters to be presentat various concentrations at different times of the reaction. These mayrepresent the active rhodium carbonyl species responsible for polyhydricalcohol formation or may be merely symptomatic of some furtherintermediate transitory rhodium carbonyl structure which serves toconvert the carbon monoxide and hydrogen to the polyhydric alcohol.

A number of nitrogen and/or oxygen-containing bases may also be used inthe process of the present invention. For the purposes of thisinvention, the bases can be considered to promote the activity of therhodium catalyst.

Nitrogen Lewis bases used as promoters generally contain hydrogen andnitrogen atoms. They may also contain carbon and/or oxygen atoms. Theymay be organic or inorganic compounds. With respect to the organiccompounds, the carbon atoms can be part of an acyclic and/or cyclicradical such as aliphatic, cycloaliphatic, aromatic (including fused andbridged) carbon radicals, and the like. Preferably, the organic Lewisbases contain from 2 to 60, most preferably 2 to 40 carbon atoms. Thenitrogen atoms can be in the form of imino (--N═), amino (--N--),nitrilo (N═), etc. Desirably, the Lewis base nitrogen atoms are in theform of imino nitorgen and/or amino nitrogen. The oxygen atoms can be inthe form of groups such as hydroxyl (aliphatic or phenolic), carboxyl##STR1## carbonyloxy ##STR2## oxy (--O--), carbonyl ##STR3## etc., allof said groups containing Lewis base oxygen atoms. In this respect, itis the "hydroxyl" oxygen in the ##STR4## group and the "oxy" oxygen inthe ##STR5## group that are acting as Lewis base atoms. The organicLewis bases may also contain other atoms and/or groups as substituentsof the aforementioned radicals, such as alkyl, cycloalkyl, aryl, chloro,trialkylsilyl substituents.

Illustrative of organic aza-oxa Lewis bases are, for example, thealkanolamines, such as, ethanolamine, diethanolamine, isopropanolamine,di-n-propanolamine, and the like; N,N-dimethylglycine,N,N-diethylglycine; iminodiacet acid, N-methyliminodiacetic acid;N-methyldiethanolamine; 2-hydroxypyridine, 2,4-dihydroxypyridine,2-methoxypyridine, 2,6-dimethoxypyridine, 2-ethoxypyridine; lower alkylsubstituted hydroxypyridines, such as 4-methyl-2-hydroxypyridine,4-methyl-2,6-dihydroxypyridine, and the like; morpholine, substitutedmorpholines, such as 4-methylmorpholine, 4-phenylmorpholine; picolinicacid, methyl-substituted picolinic acid; nitrilotriacetic acid,2,5-dicarboxypiperazine N-(2-hydroxyethyl) iminodiacetic acid,ethylenediamine-tetraacetic acid; 2,6-dicarboxypyridine;8-hydroxyquinoline, 2-carboxyquinoline,cyclohexane-1,2-diamine-N,N,N',N'-tetraacetic acid, the tetramethylester of ethylenediamine-tetraacetic acid, and the like.

Other Lewis base nitrogen containing compounds include organic andinorganic amines.

Illustrative of such inorganic amines are, e.g., ammonia, hydroxylamine,and hydrazine. Primary, secondary, or tertiary organic amines arepromoters. This includes the mono- and polyamines (such as di-, tri-,tetraamines, etc.) and those compounds in which the Lewis base nitrogenforms part of a ring structure as in pyridine, quinoline, pyrimidinemorpholine, hexamethylenetetraamine, and the like. In addition anycompounds capable of yielding an amino nitrogen under the reactionconditions of the present invention are promoters, as in the case of anamide, such as formamide, cyanamide, and urea, or an oxime. Furtherillustrative of Lewis base nitrogen compounds are aliphatic amines suchas methylamine, ethylamine, n-propylamine, isopropylamine, octylamine,dodecylamine, dimethylamine, diethylamine, diisoamylamine,methylethylamine, diisobutylamine, trimethylamine, methyldiethylamine,triisobutylamine, tridecylamine, and the like; aliphatic and aromaticdi- and polyamines such as 1,2-ethanediamine, 1,3-propanediamine,N,N,N',N'-tetramethylenediamine, N,N,N',N'-tetraethylethylenediamine,N,N,N',N'-tetra-n-propylethylenediamine,N,N,N',N'-tetrabutylethylenediamine, o-phenylenediamine,m-phenylenediamine, p-phenylenediamine, p-tolylenediamine, o-tolidene,N,N,N',N'-tetramethyl-p-phenylenediamine,N,N,N',N'-tetraethyl-4,4'-biphenyldiamine, and the like; aromatic aminessuch as aniline, 1-naphthylamine, 2-naphthylamine, p-toluidine,o-3-xylidine, p-2-xylidine, benzylamine, diphenylamine, dimethylaniline,diethylaniline, N-phenyl-1-napthylamine,bis-(1,8)-dimethylaminonaphthalene, and the like; alicyclic amines suchas cyclohexylamine, dicyclohexylamine, and the like; heterocyclic aminessuch as piperidine; substituted piperidines such as 2-methylpiperidine3-methylpiperidine, 4-ethylpiperidine, and 3-phenylpiperidine; pyridine;substituted pyridines such as 2-methylpyridine, 2-phenylpyridine,2-methyl-4-ethylpyridine, 2,4,6-trimethylpyridine, 2-dodecylpyridine,2-chloropyridine, and 2-(dimethylamino) pyridine; quinoline; substitutedquinolines, such as 2-(dimethylamino)-6-methoxyquinoline;4,5-phenanthroline; 1,8-phenanthroline; 1,5-phenanthroline; piperazine;substitute piperazines such as N-methylpiperazine, N-ethylpiperazine,2-methyl-N-methylpiperazine; 2,2'-dipyridyl, methyl-substituted2,2-dipyridyl; ethyl-substituted 2,2'-dipyridyl;4-triethylsilyl-2,2'-dipyridyl; 1,4-diazabicyclo[2.2.2]octane, methylsubstituted 1,4-diazabicyclo[2.2.2]octane, purine and the like.

Also included herein are the use of dimorpholine compounds characterizedby the formula: ##STR6## wherein R is divalent alkylene of 1 to about 30carbon atoms and 1,4-phenylene.

Under reaction conditions the promoter is preferably used in amountsfrom about 0.02 to about 40 equivalents of promoter, most preferablyfrom about 0.1 to about 20 equivalents promoter, for every atom ofrhodium in the reaction mixture. The number of equivalents of promoteris equal to the number of molecules of promoter times the number ofnitrogen atoms in each molecule.

The process of this invention can also be carried out by providing saltsin the homogeneous liquid phase reaction mixture. Suitable salts includeany organic or inorganic salt which does not adversely affect theproduction of polyhydric alcohols. Experimental work suggest that manysalts are beneficial as either a copromoter and/or in aiding inmaintaining rhodium in solution during the reaction. Illustrative ofuseful salt promoters are the ammonium salts and the salts of the metalsof Group I and Group II of the Periodic Table (Handbook of Chemistry andPhysics--50th Edition) for instance the halide, hydroxide, alkoxide,phenoxide and carboxylate salts such as sodium fluoride, cesiumfluoride, cesium pyridinolate, cesium formate, cesium acetate, cesiumbenzoate, cesium p-methylsulfonyl-benzoate (CH₃ SO₂ C₆ H₄ COO)Cs,rubidium acetate, magnesium acetate, strontium acetate, ammoniumformate, ammonium benzoate and the like. The cesium, rubidium, potassiumand ammonium salts are preferred.

Other organic salts useful in the practice of the present inventioninclude the quaternized heterocyclic amine salts such as the pyridinium,piperidinium, morpholinium, quinolinium salts and the like, e.g.,N-ethylpyridinium fluoride, N-methylmorpholinium benzoate,N-phenylpiperidinium hydroxide, N,N'-dimethyl-2,2-bipyridinium acetate,and the like.

In addition, the anion of the above salt may be any of the rhodiumcarbonyl anions. Suitable rhodium carbonyl anions include [Rh₆ (CO)₁₅]²⁻ ; [Rh₆ (CO)₁₅ Y]⁻ wherein Y may be halogen, such as chlorine,bromine, or iodine, [Rh₆ (CO)₁₅ (COOR"]⁻ wherein R" is lower alkyl oraryl such as methyl, ethyl, or phenyl; [Rh₆ (CO)₁₄ ]²⁻ ; [Rh₇ (CO)₁₆ ]³⁻; [Rh₁₂ (CO)₃₀ ]²⁻ ; Rh₁₃ (CO)₂₄ H₃ ⁻² ; and Rh₁₃ (CO)₂₄ H₂ ⁻³.

Under reaction conditions where a salt is employed the salt is desirablyadded with the initial charge of reactants in amounts of from about 0.5to about 2.0 gram moles, preferably from about 0.8 to about 1.6 moles,and most preferably from about 0.9 to 1.4 moles of salt for every fivegram atoms of rhodium present in the reaction mixture.

Illustrative solvents which are generally suitable in making thehomogeneous mixture include, for example, ethers such astetrahydrofuran, tetrahydropyran, diethyl ether, 1,2-dimethoxybenzene,1,2-diethoxybenzene, the mono- and dialkyl ethers of ethylene glycol, ofpropylene glycol, of butylene glycol, of diethylene glycol, ofdipropylene glycol, of triethylene glycol, of tetraethylene glycol, ofdibutylene glycol, of oxyethylenepropylene glycol, etc; alkanols such asmethanol, ethanol, propanol, isobutanol, 2-ethylhexanol, etc.; ketonessuch as acetone, methyl ethyl ketone, cyclohexanone, cyclopentanone,etc.; esters such as methyl acetate, ethyl acetate, propyl acetate,butyl acetate, methyl propionate, ethyl butyrate, methyl laurate, etc.;water; gamma-butyrolactone, deltavalerolactone; substituted andunsubstituted tetrahydrothiophene-1,1-dioxides (sulfolanes) as disclosedin U.S. application Ser. No. 537,885, filed on Jan. 2, 1975, thedisclosure at pages 6 and 7 of the specification of which isincorporated herein by reference.

Also, the crown ethers are suitable herein, particularly those asdescribed in U.S. Patent Application Ser. No. 832,384 filed Sept. 13,1977, which application is incorporated herein by reference and is nowU.S. Pat. No. 4,162,261 issued July 24, 1979. The crown ethers describedtherein contain at least four oxygen heteroatoms and include[18]-crown-6 and [15]-crown-5.

Particularly desirable solvents are tetraglyme, sulfolane,gamma-butyrolactone and the crown ethers. Other very desirable solventsinclude mixtures of tetraglyme and sulfolane, mixtures of sulfolane andbutyrolactone, mixtures of crown ethers and sulfolane, mixtures of crownethers and tetraglyme, mixtures of crown ethers and butyrolactone,mixtures of tetraglyme and butyrolactone.

In practicing the method of the present invention, the synthesis of thedesired alkane diols and derivatives thereof, by the reaction ofhydrogen with an oxide of carbon is suitably conducted under operativeconditions, as heretofore described, which give reasonable reactionrates and/or conversions.

The process is suitably effected over a wide superatmospheric pressurerange of from about 500 psia to about 50,000 psia. Pressures as high as50,000 psia, and higher can be employed but with no apparent advantagesattendant thereto which offset the unattractive plant investment outlayrequired for such high pressure equipment. Therefore, the upper pressurelimitation is desirably approximately 16,000 psia. Effecting the presentprocess below about 16,000 psia, especially below about 13,000 psia, andpreferably at pressures below about 8000 psia, results in costadvantages which are associated with low pressure equipmentrequirements. In attempting to foresee a commercial operation of thisprocess, pressures between about 4,000 psia and 16,000 psia appear torepresent most realistic values.

In a preferred embodiment of the present invention the pressuresreferred to above represent the total pressures of hydrogen and oxidesof carbon in the reactor.

The temperature which may be employed can vary over a wide range ofelevated temperatures. In general, the process can be conducted at atemperature in the range of from about 100° C. and upwards toapproximately 375° C., and higher. Temperatures outside this statedrange are not excluded from the scope of the invention. At the lower endof the temperature range, and lower, the rate of reaction to desiredproduct becomes markedly slow. At the upper temperature range, andbeyond, signs of some catalyst instability are noted. Notwithstandingthis factor, reaction continues and alkane polyols and/or theirderivatives are produced. Additionally, one should take notice of theequilibrium reaction for forming ethylene glycol:

    2CO+3H.sub.2 ⃡HOCH.sub.2 CH.sub.2 OH

At relatively high temperatures the equilibrium increasingly favors theleft hand side of the equation. To drive the reaction to the formationof increased quantities of ethylene glycol, higher partial pressures ofcarbon monoxide and hydrogen are required. Processes based oncorrespondingly higher pressures, however, do not represent preferredembodiments of the invention in view of the high investment costsassociated with erecting chemical plants which utilize high pressureutilities and the necessity of fabricating equipment capable ofwithstanding such enormous pressures. Suitable temperatures are betweenabout 150° C. to about 320° C., and desirably from about 210° C. toabout 300° C.

The novel process is effected for a period of time sufficient to producethe alkane polyols and/or derivatives thereof. In general, the residencetime can vary from minutes to several hours, e.g., from a few minutes toapproximately 24 hours, and longer. It is readily appreciated that theresidence period will be influenced to a significant extent by thereaction temperature, the concentration and choice of the catalyst, thetotal gas pressure and the partial pressures exerted by its components,the concentration and choice of diluent, and other factors. Thesynthesis of the desired product(s) by the reaction of hydrogen with anoxide of carbon is suitably conducted under operative conditions whichgive reasonable reaction rates and/or conversions.

The relative amounts of carbon monoxide and hydrogen which are initiallypresent in the reaction mixture can be varied over a wide range. Ingeneral, the mole ratio of CO:H₂ is in the range of from about 20:1 toabout 1:20, suitably from about 10:1 to about 1:10, and preferably fromabout 5:1 to about 1:5.

It is to be understood, however, that molar ratios outside theaforestated broad range may be employed. Substances or reaction mixtureswhich give rise to the formation of carbon monoxide and hydrogen underthe reaction conditions may be employed instead of mixtures comprisingcarbon monoxide and hydrogen which are used in preferred embodiments inthe practice of the invention. For instance, polyhydric alcohols areobtained by using mixtures containing carbon dioxide and hydrogen.Mixtures of carbon dioxide, carbon monoxide and hydrogen can also beemployed. If desired, the reaction mixture can comprise steam and carbonmonoxide.

The active forms of the rhodium carbonyl clusters may be prepared byvarious techniques. They can be preformed and then introduced into thereaction zone. Alternatively, any of the host of rhodium-containingsubstances can be introduced into the reaction zone and, under theoperative conditions of the process (which of course includes hydrogenand carbon monoxide), the active rhodium carbonyl cluster can begenerated in situ. Illustrative of rhodium-containing substances whichcan be conveniently introduced or placed in the synthesis zone include,for example, rhodium oxide (Rh₂ O₃), tetrarhodium dodecacarbonyl,dirhodium octacarbonyl, hexarhodium hexadecacarbonyl (Rh₆ (CO)₁₆),rhodium(II) formate, rhodium(II) acetate, rhodium(II) propionate,rhodium(II) butyrate, rhodium(II) valerate, rhodium(III) naphthenate,rhodium dicarbonyl acetylacetonate, rhodium tri(acetylacetonate),rhodium trihydroxide, indenylrhodium dicarbonyl, rhodium dicarbonyl(1-phenylbutane-1,3-dione), tris(hexane-2,4-dionato)rhodium(III),tris(heptane-2,4-dionato)rhodium(III),tris(1-phenylbutane-1,3-dionato)rhodium(III),tris(3-methylpentane-2,4-dionato)rhodium(III),tris(1-cyclohexylbutane-1,3-dionato)rhodium(III), triacontacarbonylrhodium salts and rhodium-containing compounds deposited on poroussupports or carriers capable of providing rhodium carbonyls in solution,and others.

The preparation of the rhodium carbonyl complex compounds can beconveniently carried out in the solvent mixture. Tetrarhodiumdodecacarbonyl, though of limited solubility, can be added to thesolvent in a finely divided form. Any of several of therhodium-containing compounds illustrated previously can be employed inlieu of tetrarhodium dodecacacarbonyl. The rhodium carbonyl complex orcluster forming reaction can be effected under a carbon monoxidepressure, with or without H₂, of about 1 to 15 atmospheres, and higher,using a temperature of about 30° C. to about 100° C., for a period oftime ranging from minutes to a few days, generally from about 30 minutesto about 24 l hours. The resulting rhodium carbonyl complex contained inthe solvent mixture is catalytically active in this process.

In order to illustrate the invention reference is made to the drawingwhich depicts a schematic flowsheet describing a liquid recycle systemfor the continuous operation of the process of the invention.

Reactor 2 is a heat jacketed stainless steel autoclave capable ofwithstanding reaction pressures of up to 2,000 atmospheres. Provided inreactor 2 is the liquid phase mixture containing the rhodium carbonylcomplex catalyst dissolved therein. Hydrogen and carbon monoxide aremixed in the desired molar ratio using a metering system (not shown)which allows the composition of the exit gas to vary from pure H₂ topure CO. When reactor 2 is operated in an overflow (or liquid full)configuration, the resultant gas feed is passed through gas compressor 1and fed to reactor 2 in quantities sufficient to saturate the liquidphase mixture in the reactor but less than an amount whereby a discretegas phase will form in the reaction zone. Reactor 2 is provided with anagitator to uniformly distribute the H₂ /CO gas mixture in the liquidbody thus providing intimate gas-liquid contact.

In operating reactor 2 in an overflow (or liquid full) configuration,the liquid phase mixture comprising the catalyst solution, reactionproducts and dissolved reactant gas is withdrawn from the top of reactor2 and is passed through pressure reducing value 10 to a liquid-vaporseparator 3 where a portion of the unconverted reactant gas dissolved inthe liquid mixture comes out of solution at the reduced pressure of theseparator and is vented through line 6. The liquid mixture leavingseparator 3 is either returned directly to reactor 2 via line 7,pressure reducing valve 12 and liquid pump 4, or alternatively, sentthrough line 8, and pressure reducing valve 11 to CO stripper 5 forrecovery of product, predominantly ethylene glycol with varying amountsof polyols such as glycerine, propylene glycol and methanol. Stripper 5is a vertical packed column filled with a high surface area packing, theliquid feed entering near the top being stripped by a counter-currentflow of carbon monoxide or optionally a CO/H₂ mixture which is thenfurther processed for product recovery. The liquid mixture exiting atthe bottom of stripper 5 is recycled to reactor 2 through line 9, valve13 and pump 4. Valve 13 prevents the flow of liquid mixture leavingvalve 12 from entering stripper 5; valve 13 being closed when the liquidmixture in line 7 bypasses stripper 5.

As an alternative to operating reactor 2 in an overflow configuration,as described above, reactor 2 may operate as a vapor-stripping reactorin a gas recycle system. In this mode of operation, the liquid level ofcatalyst solution in reactor 2 is controlled such that a liquid-vaporinterface is maintained in the reactor, the vapor space above the liquidphase mixture being composed of excess reactant gas and reactionproduct. An excess amount of H₂ and CO reactant gas mixture is thusintroduced into the bottom of reactor 2 to contact the catalyst solutiontherein and strip therefrom vaporous products of the reaction andsolvent. The CO/H₂ gas mixture in the vapor space is thus saturated withvaporous reaction products and solvent vapors and passes from the top ofreactor 2 into vapor-liquid separator 3. For a gas recycle system, theoperating pressure of separator 3 is preferably maintained at thepressure of reactor 2. A heat exchanger may be provided wherein all butthe CO/H₂ mixture is liquified and removed from line 7, the CO/H₂mixture being withdrawn through line 6 and recycled to reactor 2 (gasrecycle not shown in drawing). The consensed liquid mixture in line 7 isthen sent to stripper 5, as previously described, for recovery ofproduct, the remaining liquid in stripper 5 exiting through line 9 forrecycle to reactor 2.

Resolubilization of rhodium precipitated from the catalyst solution maybe effected by periodically reducing the temperature of reactor 2 aspreviously described. This is conveniently accomplished by allowing thereactor to cool to a predetermined lower temperature, maintaining thecatalyst solution at the lower temperature for the requisite amount oftime necessary to effect at least a partial resolubilization ofprecipitated rhodium and thereafter raising the reactor temperature toabout its former value. Alternatively, the temperature of reactor 2 maybe maintained constant. In accordance with this embodiment of theinvention, a portion of the liquid phase mixture is withdrawn fromreactor 2 on a periodic or continuous basis via line 17 and liquid pump14, cooled in heat exchanger 15 and then maintained at such lowertemperature for the desired residence time by adjusting the flow rate ofsolution in line 17, or by providing a holding tank (not shown) forretaining the cooled catalyst solution, the solution being thereafterreheated to the reaction temperature in heat exchanger 16 prior to beingreintroduced into reactor 2. If desired, the catalyst solution cooled inheat exchanger 15 may be heated to the reaction temperature in reactor 2thus eliminating the need for heat exchanger 16.

To further illustrate the invention, the following examples wereoperated in accordance with the liquid recycle system illustrated inFIG. 1. Reactor 2 was operated in a liquid-full configuration. In thespecific practice of the examples, a 30 or 60 pound charge of solution,as the case may be, containing solvent, rhodium dicarbonylacetylacetonate, and promoters was initially introduced into a holdingtank (not shown in the drawing) under nitrogen pressure. A 1:1 molemixture of H₂ /CO, at the desired gas feed rate, was introduced into a1.5 or 6 liter capacity stainless steel reactor (depending upon theparticular example) until the desired reactor pressure was establishedin the system. The catalyst solution was then charged from the holdingtank into the reactor and the reactor then heated to the desiredreaction temperature.

The dimensions of the 1.5 and 6 liter reactors used in the examples wereas follows: 35/8" I.D.×9" and 51/2" I.D.×18". The liquid-vapor separatorwas a 2 liter capacity stainless steel vessel, 3" I.D., 18" long. The COstripper was a 2" I.D. tube, 8 ft. long.

The separator was operated at about 2,000 psig and the pressure of theCO stripper was about 15 psig.

EXAMPLE I

A 60 pound charge of catalyst solution containing about 250 ppm byweight of rhodium (added as rhodium dicarbonyl acetylacetonate) andhaving a 6 to 1 molar ratio of rhodium to cesium (50 ppm added as cesiumfluoride) and a 4:1 molar ratio of 2-hydroxypridine to rhodium intetraglyme solvent was introduced into a liquid recycle system, asdescribed above, having a 6 liter stainless steel reactor. The feed gaswas a 1:1 mole ratio of H₂ /CO, the reactor temperature was 210° C. andthe reactor pressure was 10,000 psig. After 150 hours of operation, therhodium concentration in solution had declined to about 150 ppm whilethe cesium concentration was about 30 ppm. The reactor temperature wasthen lowered to 100° C. without changing any of the other reactionvariables. After 40 hours of operation the rhodium concentration hadincreased to about 500 ppm and the cesium concentration to about 60 ppm.The apparent resolubilization of rhodium in amounts greater than thatwhich was lost or precipitated from solution is believed to be due tothe resolubilization of rhodium which was precipitated from othercatalyst charges during the course of prior experiments conducted in thesame reactor.

EXAMPLE II

An experiment was carried out using a catalyst solution similar involume, composition and mole ratios of rhodium to promoters to thatdescribed in Example 1 except that the rhodium concentration was about320 ppm by weight and the cesium concentration was about 70 ppm. Thereaction temperature, pressure, and H₂ /CO gas mixture were all asdescribed in Example 1. The reactor volume was 6 liters. After 76 hoursof reaction, the rhodium concentration had declined to about 170 ppm andcesium to about 55 ppm whereupon the reaction temperature was reduced to100° C. and nitrogen was substituted for the H₂ /CO feed gas. After 12hours at these conditions the rhodium concentration was still atapproximately 170 ppm. The cesium concentration was not determined. Thefeed gas was then switched to a 1:1 mole ratio of H₂ /CO whilemaintaining all other reaction conditions constant. After 12 hours ofoperation, the rhodium concentration was about 330 ppm and the cesiumconcentration about 67 ppm.

The substitution of N₂ for the H₂ /CO feed, as described above, did notresult in any measured increase in rhodium concentration and appears toindicate that the resolubilization of rhodium at a lowered reactiontemperature in accordance with the invention does not occur when thecatalyst solution is under an inert gas pressure rather than that of theH₂ /CO reactant gas.

EXAMPLE III

A catalyst solution was used similar in volume, composition and moleratios of rhodium to promoters to that described in Example 1 exceptthat the rhodium concentration was about 830 ppm. The reactiontemperature was 220° C. while the reaction pressure and H₂ /CO feed gaswere as set forth in Example 1. The reactor volume was 6 liters. After168 hours of operation, the concentration of rhodium in solution haddeclined to about 100 ppm. The reactor temperature was then decreased toabout 100° C. and the liquid circulation in the system was stopped whilemaintaining the H₂ /CO feed. After 24 hours at these conditions thecatalyst solution in the reactor was sampled and found to contain inexcess of 5600 ppm rhodium. The circulation of solution was thenrestarted and after becoming well mixed, once again, the solution wasanalyzed and the concentration of rhodium in solution determined to beabout 830 ppm.

The measured concentration of rhodium above 5600 ppm, as describedabove, appears to indicate that the original loss of rhodium fromsolution and subsequent resolubilization occurred predominantly in thereactor and not elsewhere in the system.

EXAMPLE IV

A 30 lb. charge of catalyst solution containing about 112 grams ofrhodium (added as rhodium dicarbonyl acetylacetonate), potassium (addedas potassium bicarbonate) in a 5:1 molar ratio of RH/K andN-methylmorpholine at a concentration of about 1.5 weight % in a solventconsisting of the following: 18-Crown-6 (about 75 weight %) andtriphenyl phosphine oxide (about 20 weight %), with the balance beingreaction products, was introduced into a liquid recycle system having a1.5 liter stainless steel reactor. The feed gas was a 0.5:1 mole ratioof hydrogen to carbon monoxide, the reaction temperature was 265° C. andthe reaction pressure was about 15,000 psig. The initial productivity ofethylene glycol was about 4.5 gram-moles per hour. After about 100 hoursof operation, the amount of rhodium in solution had decreased to about78 grams, the productivity declining to about 3.5 gram-moles of glycolper hour. The reactor temperature was then reduced to 230° C. whilemaintaining all other reaction conditions constant. After 6 hours ofoperation the weight of rhodium in solution had increased about 110grams. After an interval during which another experiment was conductedwith the catalyst system which involved decreasing the molar ratio ofRh/K to about 2.5:1, the temperature was, once again, elevated to 265°C. The initial productivity was thereafter about 4.2 gram-moles perhour.

What is claimed is:
 1. In a process for producing alkane polyols in areaction zone by the continuous reaction of hydrogen and oxides ofcarbon in a homogeneous liquid phase mixture containing a rhodiumcarbonyl complex catalyst at a temperature of from about 100° C. toabout 375° C. and a pressure of from about 500 psia to about 50,000psia, the improvement which comprises periodically lowering thetemperature of said liquid phase mixture and thereafter raising it toabout its former value.
 2. The process of claim 1 wherein in saidimprovement the liquid phase mixture has its temperature periodicallylowered and raised while in the reaction zone.
 3. The process of claim 1wherein in said improvement the reaction temperature is lowered by about10° C. to about 30° C.
 4. The process of claim 1 wherein in saidimprovement a portion of said liquid phase mixture has its temperatureperiodically lowered outside the reaction zone and is then returned tosaid reaction zone.
 5. The process of claim 4 wherein said portion iscontinuously withdrawn from the reaction zone.
 6. The process of claim 5wherein said portion has its temperature lowered on a continuous basis.7. The process of claim 4 wherein the temperature is lowered by about10° C. to about 30° C. outside of the reaction zone.
 8. The process ofclaim 4 wherein said portion of the liquid phase mixture has itstemperature raised to about its former value prior to being reintroducedinto the reaction zone.